Accelerometers generally measure acceleration forces applied to a body. Accelerometers are typically mounted directly onto a surface of the accelerated body. Such direct mounting ensures the immediate detection of even subtle forces exerted on the body. The directly mounted accelerometer is, however, also exposed to various extraneous shock, vibration and thermal stresses experienced by the accelerated body. The accelerometer measures the forces induced by such external stresses in combination with the applied acceleration forces and renders confused and inaccurate acceleration measurements. Generally, isolation mechanisms between the accelerometer and the accelerated body, typically integral to the accelerometer housing, protect the accelerometer from forces induced by stresses within the accelerated body.
While stresses experienced by the accelerated body are isolated from the accelerometer, sensitive accelerometers can suffer from error sources caused by subtle forces induced by stresses internal to the accelerometer but external to the acceleration sensing mechanism. In monolithic micro-machined accelerometers having vibrating beam force detectors suspended between a movable proof mass and an accelerometer frame, such forces are caused by, for example, bonding stresses between a silicon cover plate and the sensor frame or other assembly stresses. Other such forces are caused by, for example, thermal stresses resulting from a mismatch of thermal expansion coefficients between materials within the sensor. External thermal stresses may be induced by the typical mechanical coupling of the sensor frame to the silicon cover plate and by the mechanical coupling of the silicon cover plate to a ceramic or metal mounting plate. Because the cover and mounting plates are typically fabricated from different materials, they usually have substantially different coefficients of thermal expansion. When heated, the mismatch in thermal expansion coefficients generally causes undesirable stresses which induce distortion and strain in the sensor frame.
Bias performance and stability of monolithic silicon based accelerometers is based on proof mass sizing, commonly referred to as pendulousity, and on the degree of stress isolation in the mechanical die stack. Monolithic micro-machined vibrating beam accelerometers are typically targeted for small size which limits the proof mass size and generally requires special care in providing isolation from external stresses. Historically, the accelerometer frame is suspended from a second outer frame by flexures that permit the accelerometer frame to move relative to the outer frame, as shown and described in allowed U.S. patent application Ser. No. 08/735,299. Such isolation structure designs as have been possible using a potassium hydroxide (KOH) etching solution in a bulk process to cost effectively fabricate monolithic micro-machined vibrating beam accelerometers effectively minimize the distortion of the accelerometer frame and decrease the effects of the thermal coefficient mismatch. However, the orientation of the natural etch planes in silicon at 54.7 degrees from horizontal requires relatively large amounts of physical space when using a KOH etching solution, thus limiting both the pendulousity, i.e., possible proof mass size, and the possible isolation structure designs and requiring major compromises and trade-offs in proof mass sizing and isolation structure design in very small applications.
Furthermore, although some monolithic micro-machined vibrating beam accelerometers have included isolation structure in the one cover plate by which the sensor mechanism is mounted to the ceramic or metal mounting plate, to date, no monolithic micro-machined vibrating beam accelerometers have included isolation structure in the both cover plates for reducing or eliminating residual stresses caused by die bowing. Additionally, to date, none has provided a large centralized mounting area surrounded with a self-caging structure for surviving high shock loads.